The Norwalk virus and related 27nm viruses are an important cause of epidemic viral gastroenteritis that occurs in family, school, group, institutional or community-wide outbreaks affecting adults, school-aged children, family contacts and some young children as well. However, large scale epidemiologic studies as well as the development of routine laboratory diagnostic assays for Norwalk virus have been hindered by the absence of a readily accessible source of antigen because this virus has not yet been grown in any tissue culture system. Therefore, serologic assays have relied on antigen positive stool material. Recently, this obstacle was overcome by the production of recombinant Norwalk virus capsid protein in a baculovirus expression system (Estes), and this self- assembled virus-like particle was found to be comparable to the native virus in serologic assay. We have used the recombinant Norwalk virus antigen in an ELISA to examine the serologic responses to Norwalk virus in various populations. We have previously shown that: (i) Norwalk virus infection occurred in 6.2% of a subset of troops deployed to Saudi Arabia and Kuwait during a five-month interval; and (ii) over a period of almost 2 years, 49% of 154 infants and young children in Finland exhibited serologic evidence of Norwalk virus infection. During this reporting period, we describe the use of the recombinant Norwalk virus antigen in: (i) elucidating the role of Norwalk virus infection in an outbreak of gastroenteritis aboard a U.S. aircraft carrier, and (ii) among troops who developed gastroenteritis while deployed to Somalia.